There are currently different techniques for characterizing mechanical, tribological, physicochemical or other properties of materials, surface coatings and lubricants, which use some form of controlled mechanical action on the surface to be characterized in a special atmosphere, for example, in ultra-high vacuum or in an atmosphere of controlled gas and pressure. In these techniques, a burin or punch which exerts a pressure force in the direction normal to the surface to be characterized is normally used. Also, in some cases as the result of the traction of the burin or punch on the surface to be characterized a friction force is produced in the direction tangential to the surface to be characterized. The control of the normal force and the measurement of the tangential force with high precision and reproducibility are conditions necessary for many characterization techniques. Furthermore, to characterize different materials such as ceramics, metals, metal alloys, polymers, composite materials, hard surface coatings, solid or liquid lubricants, greases, etc., which have very different mechanical properties such as hardness, modulus of elasticity, free surface energy, etc., the device has to have a broad range of measurement.
There are several indirect methods and devices for measuring forces described in U.S. Pat. No. 5,115,664, U.S. Pat. No. 5,212,657, U.S. Pat. No. 7,000,451, in Stachowiak G. Experimental Methods in Tribology.—Amsterdam: Elsevier, 2004; and in Liu H. Bhushan B. Adhesion and friction studies of microelectromechanical systems/nanoelectromechanical systems materials using a novel microtriboapparatus. J. Vac. Sci. Technol. A21(4), 2003, pp. 1528-1538. These methods use a flexible member with a well determined stiffness constant. This flexible member is usually located between a stiff base and the sample or between a stiff base and the punch or burin which exerts force on the sample. The normal force exerted by the burin or punch on a surface is determined by the deflection of the flexible member in the direction normal to the surface of the sample. The tangential force resulting from the friction between the burin or punch and the surface to be characterized is determined by the deflection of the flexible member in the direction tangential to the surface. In both cases it is necessary to determine the stiffness constants of the flexible member in every direction by means of prior calibration of a sensor.
In the devices described in the previously mentioned patents and publications, the deflection of the flexible member is measured by means of a sensor, which by way of illustration can be one of the following types: fiber optic, capacitive, inductive, laser interferometric sensor or the like. One of the most widely used ones is the fiber optic sensor. In the mentioned devices, the distance between the sensor and the flexible member is pre-established before creating the vacuum or controlled atmosphere and cannot be changed or adjusted without breaking the vacuum. In that of the publication of Liu H., Bhushan B. Adhesion and friction Studies of microelectromechanical systems/nanoelectromechanical systems materials using a novel microtriboapparatus. J. Vac. Sci. Technol. A21(4), 2003, pp. 1528-1538, a fiber optic sensor is used in which the distance between the end of an optical fiber and a reflective surface can be adjusted by means of a manually operated external micropositioner. This external micropositioner allows selecting a near range or a far range, depending on the necessary measurement resolution. In this device there are two piezoelectric motors used for moving the sample in two coordinates. However, these piezoelectric motors are not used for the movement and adjustment of the fiber optic sensors.
The manual adjustment of the position of the sensors by means of an external positioner is a common feature of all previously disclosed devices and constitutes a significant obstacle when performing the fine adjustment of the position of the sensor or calibrating it again in vacuum or controlled atmosphere applications when, in order to perform this adjustment or calibration, it is necessary to break the vacuum or controlled atmosphere. In these applications it is not possible to adjust the position or calibrate the sensors used for measuring the deflection of the flexible members remotely. In ultra-high vacuum systems which require heating of the system for the degassing thereof at a temperature normally comprised between 100° C. and 400° C., deformations of the structural members of the system may occur as the result of the high temperature or the pressure. This can affect the position of the sensors installed within the system, cause their misalignment, be detrimental to the measurement and, in some cases, prevent their use for measuring forces. In existing systems, in order to adjust the position of the sensors again it is necessary to open the system and break the vacuum or the controlled atmosphere, which entails losses of time (up to several days) and considerable economic costs.